An Orthogonal Frequency Division Multiplexing (OFDM) method is one of the conventional transmitting methods of a mobile radio communication system. The OFDM method is employed in a communication system such as, for example, Long Term Evolution (LTE), Super 3G, and Worldwide Interoperability for Microwave Access (WiMAX). In the OFDM method, a modulated signal is mapped in a frequency domain and is then transformed into a signal of a time domain. The signal of the time domain is transformed into a signal of the frequency domain by performing Fast Fourier Transform (FFT). For example, there is a receiving device that transforms each reception signal of a plurality of systems received by a plurality of reception antennas into a reception signal for each sub-carrier by performing the FFT. As an example of the above-described receiving device, there is a device that detects a delay time portion, which is longer than a guard interval of data, as an intersymbol interference portion, and that performs FFT processing on a result obtained by subtracting a replica of a time wave form portion of a known symbol that corresponds to the intersymbol interference portion (for example, Japanese Laid-open Patent Publication No. 2004-208254). Furthermore, there is a receiving device that generates a replica of interference components between the systems and cancels the interference components between the systems included in each reception signal by subtracting the replica from the reception signal of a plurality of systems after the FFT is performed (for example, Japanese Laid-open Patent Publication No. 2004-235916).
However, when the conventional receiving device performs the FFT on the reception signal, the sub-carriers may have different average powers for the sub-carriers after the FFT is performed. For example, if the received power of the sub-carrier allocated in a far terminal is larger than the received power of the sub-carrier allocated in a near terminal. In this case, if the number of bits of the FFT and level diagrams is not sufficiently secured, overflow occurs in the sub-carrier with the large power after the FFT is performed. On the other hand, the number of allocation bits is small in the sub-carrier with the small power, so that the accuracy of the FFT becomes lower. Regarding the sub-carrier with the large power, occurrence of overflow may be prevented by adjusting a level of an input signal to be subjected to the FFT. Even in this case, the sub-carrier with the small power may have a bad use efficiency of the number of bits, so that there is a problem that the number of bits used for the FFT is increased to achieve sufficient accuracy.